High amplitude rotational oscillations of the drillstring are a common problem while drilling. They are generated by the combination of the torque generated by the interaction of the bit with the hole-bottom and of the drillstring with the borehole walls, and the lack of damping of the rotational oscillations. One of the reasons that there is so little damping is that the bit-rock interaction does not provide any damping, and indeed can amplify the oscillations.
As explained in SPE 18049, slip-stick motion of the bottom hole assembly can be regarded as extreme, self-sustained oscillations of the lowest torsional mode, called the pendulum mode. Such a motion is characterized by finite time intervals during which the bit is non-rotating and the drill pipe section is twisted by the rotary table or top drive. When the drillstring torque reaches a certain level (determined by the static friction resistance of the bottom hole assembly), the bottom hole assembly breaks free and speeds up to more than twice the nominal speed before it slows down and again comes to a complete stop. It is obvious that such motion represents a large cyclic stress in the drill pipe that can lead to fatigue problems. In addition, the high bit speed level in the problems. In addition, the high bit speed level in the slip phase can induce severe axial and lateral vibrations in the bottom hole assembly which can be damaging to the connections. Finally, it is likely that drilling with slip-stick motion leads to excessive bit wear and also a reduction in the penetration rate. Frequency analysis of the driving torque associated with torsional drillstring vibrations, in particular slip-stick oscillations, reveals that a large number of torsional drillstring resonances. The sharpness of the curve at the drillstring resonance frequencies suggest there is little damping of torsional drillstring vibrations. Halsey, Kyllingstad, and Kylling, “Torque Feedback Used to Cure Slip-Stick Motion,” SPE 18049, 1988. The authors proposed a speed correction proportional to the torque to control rotational vibrations.
In WO 2014/147575 (assigned to Schlumberger), a method is described for controlling a drilling system comprising a drive system, drillstring and drill bit. The drive system rotates the drillstring during a drilling process to drill a borehole through an earth formation. The method involves setting a desired rotation speed v0 for the drillstring; receiving property measurements of the drilling system and deriving therefrom the component vup of the rotation speed of the drillstring associated with upgoing rotational energy; determining a rotation speed v for the drillstring by optimizing an expression which reconciles two conflicting objectives of: (i) maintaining a stable rotation speed centered on v0, and (ii) minimizing the downgoing rotational energy, the optimized expression expressing v in terms of v0 and vup; and controlling the drive system to rotate the drillstring at v. (See WO 2014/147575, abstract).
In U.S. Pat. No. 5,507,353 (assigned to Institut Francais du Petrole), a method and system are described for controlling the behavior of a drill bit that includes an additional resistant torque added to the torque about the drill bit so that the overall torque about the drill bit is an increasing function of the rotary speed of the bit. The system includes control means suited for creating an additional resistant torque about the bit. (See U.S. Pat. No. 5,507,353, abstract). In particular, the patent teaches to change the weight applied to the bit downhole in response to the measurements of the downhole rotation speed.
In U.S. Pat. No. 8,136,610 (assigned to Schlumberger), a method and system are described for drilling a borehole through a medium with a drill bit, a processor, and a controller. The drill bit may be configured to rotate in the medium and remove at least a portion of the medium. The processor may be configured to receive a first set of data representative of a variable rotational speed of the drill bit during a length of time in the medium, and determine, based at least in part on the first set of data, a first resonant frequency of the variable rotational speed of the drill bit. The controller may be configured to receive a second set of data representative of the first resonant frequency of the variable rotational speed of the drill bit, and vary the force applied to the drill bit based at least in part on the second set of data. (See U.S. Pat. No. 8,136,610, abstract). In particular, this patent teaches to avoid exciting the oscillations by filtering the auto-driller control signal to avoid the resonant frequency.
Notwithstanding these prior art technical developments, there is a need for a method and system that reduces or dampens torsional drillstring vibrations, in particular slip-stick oscillations and torsional drillstring resonances.